1. Field of the Invention
This invention relates to new pure-colored iron oxide direct red pigments, to a process for their production and to their use for pigmenting lacquers, plastics, building materials and ceramics.
2. Description of the Prior Art
There are four known processes for the production of red iron oxide pigments (T. C. Patton, Pigment Handbook, Vol. 1, New York 1988, page 288). One of these methods is the direct precipitation of red iron oxides which is described in U.S. Pat. No. 2,716,595. In this method, an iron(II) salt solution and an alkali solution are mixed in substantially equivalent quantities and air is passed through the iron(II) hydroxide or carbonate suspension obtained. The iron(III) oxide hydroxide seed suspension thus formed is made up into the red pigment in the presence of iron(II) salt by addition of metallic iron, heating and oxidation with oxygen-containing gases.
It is known from DE-B 1 084 405 that red pigments can only be produced from red seed suspensions and yellow pigments from yellow seed suspensions. Accordingly, the color of the end product is clearly determined by the seeds used.
Mixtures of yellow and red iron oxide pigments only give brown and unattractive color tones. U.S. Pat. No. 3,946,103 discloses a process using seed modifiers which enables pure red seed suspensions consisting of fine delta-FeOOH particles to be produced.
By virtue of their softness, the pigments produced by this method show improved properties in regard to dispersibility, gloss and flocculation behavior, particularly when used in lacquer systems, by comparison with the pigments produced by calcination of yellow and black iron oxides. In lacquer applications, however, the color purity of the iron oxide red pigments produced by direct precipitation in accordance with U.S. Pat. No. 3,946,103 does not approach that of the red pigments produced by calcination of black and yellow iron oxides.
Moreover, the pigments produced from those seed suspensions show inadequate purity of color so that the pure-colored aesthetic brick red tones required by the market for building materials cannot be achieved.
Accordingly, the problem addressed by the present invention was to provide improved iron oxide direct red pigments which would have the outstanding performance properties described above, but not the disadvantage of inadequate color purity.
Moreover, another problem addressed by the present invention was to provide improved iron oxide direct red pigments which would lead to pure-colored aesthetic brick red tones in building materials.